Runtime environments

A runtime system, also called run-time system, primarily implements portions of an execution model. This is in contrast to the runtime lifecycle phase of a program, during which the runtime system is in operation. Most languages have some form of runtime system, which implements control over the order in which work that was specified in terms of the language gets performed. Over the years, the meaning of the term 'runtime system' has been expanded to include nearly any behaviors that are dynamically determined during execution.

Every programming language specifies an execution model, and many implement at least part of that model in a runtime system. Runtime system behavior is, arguably, defined as any behavior not directly attributable to the program itself. This definition includes, as part of the runtime system, things such as putting parameters onto the stack before a function call, the behavior of disk I/O, and parallel execution related behaviors.

By this definition, essentially every language has a runtime system, including compiled languages, interpreted languages, and embedded domain-specific languages. Even API invoked stand alone execution models such as Pthreads have a runtime system that is the implementation of execution model's behavior.

Most scholarly papers on runtime systems focus on the implementation details of parallel runtime systems. A notable example of a parallel runtime system is that of Cilk, a popular parallel programming model. In addition, the proto-runtime toolkit was created to simplify the creation of parallel runtime systems.

In addition to the execution model behavior, a runtime system may also perform support services such as type checking, debugging, or code generation and optimization.

The runtime system is also the gateway by which a running program interacts with the runtime environment, which contains not only state values that are accessible during program execution, but also active entities that can be interacted with during program execution like disk drives and people via keyboards. For example, environment variables are features of many operating systems, and are part of the runtime environment; a running program can access them via the runtime system. Likewise, hardware devices such as a DVD drive are active entities that a program can interact with via a runtime system.

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